Strange vibrations emanating from a supermassive black hole appear to be growing more frequent and they could be caused by a white dwarf star orbiting perilously close to its event horizon.
In 2018, a supermassive black hole called 1ES 1927+654 took astronomers by surprise by changing from a relatively inactive black hole to an extremely bright one. It was the first time a supermassive black hole had been observed changing in this way.
The black hole began to dim after this, but now, at the Massachusetts Institute of Technology and her colleagues have found that it seems to be sending out X-ray radiation in a regular rhythm, called a quasi-periodic oscillation. They first measured these oscillations in 2022, finding that the X-ray’s brightness fluctuated by 10 per cent roughly every 18 minutes, but by 2024, this interval had decreased to every 7.1 minutes.
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“It was exciting in and of itself just to find these oscillations, because it’s only one of a handful [of supermassive black holes that do this],” says Masterson. “But I think the most exciting thing to us was that the oscillation period – how fast these oscillations were happening – was changing on human-observable timescales, which is not usually what we see around supermassive black holes.”
This changing oscillation period implies that interesting physics is happening near the black hole’s event horizon, the boundary at which its gravitational pull becomes so strong that not even light can escape.
One thing that could explain the oscillation is a white dwarf star that is occasionally having its matter siphoned off by the black hole. According to her team’s calculations, Masterson says this white dwarf would be millions of miles away from the black hole, but given that the black hole itself is millions of miles across, it puts it very close to the event horizon. “It’s actually extraordinarily close to the supermassive black hole,” says Masterson. “It’s just this tiny little object orbiting around this gargantuan beast.”
“It’s kind of amazing that such a small body can make such a significant impact on what we’re seeing from around the supermassive black hole,” she adds.
However, other scenarios could also explain the strange signal, such as a warped ring of matter called an accretion disc around the black hole, or waves of matter reverberating through the black hole’s envelope of hot gas and dust.
To confirm the white dwarf idea, astronomers will need to detect gravitational waves from the black hole. Current sensors, such as LIGO, can only detect gravitational waves at hertz or kilohertz frequencies, which correspond to black holes roughly the mass of our sun. But the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave observatory set to launch in 2035, should be capable of detecting millihertz gravitational waves, which is what supermassive black holes produce.
Knowing that a white dwarf might be orbiting a supermassive black hole is important for a mission like LISA, says at Queen’s University Belfast in the UK. “Everyone really wants to see a white dwarf get eaten by a black hole with LISA,” says Nicholl. “It helps tremendously if we have a few candidates.”
However, it is difficult to say whether the oscillation is caused by a white dwarf or one of the other explanations because of the complexity of supermassive black hole physics, says Nicholl. “The problem with black hole accretion is that we don’t really understand it, just broadly. No one really knows how accretion discs work, even though it’s a problem that’s been worked on for decades.”
arXiv
